This invention relates in general to electrostatographic developing materials, and, more particularly, to a process for coating carrier core materials.
The formation and development of images on the surface of photoconductive materials by electrostatic means is well known. The basic electrostatographic process, as taught by C. F. Carlson in U.S. Pat. No. 2,297,691, involves placing a uniform electrostatic charge on a photoconductive insulating layer, exposing the layer to a light and shadow image to dissipate the charge on the areas of the layer exposed to the light and developing the resulting electrostatic latent image by depositing on the image a finely-divided electroscopic material referred to in the art as "toner". The toner will normally be attracted to those areas of the layer which retain a charge, thereby forming a toner image corresponding to the electrostatic latent image. This powder image may then be transferred to a support surface such as paper. The transferred image may subsequently be permanently affixed to the support surface as by heat. Instead of latent image formation by uniformly charging the photoconductive layer and then exposing the layer to a light and shadow image, one may form the latent image by directly charging the layer in image configuration. The powder image may be fixed to the photoconductive layer if elimination of the powder image transfer step is desired. Other suitable fixing means such as solvent or overcoating treatment may be substituted for the foregoing heat fixing step.
Many methods are known for applying the electroscopic particles to the electrostatic latent image to be developed. One development method, as disclosed by E. N. Wise in U.S. Pat. No. 2,618,522 is known as "cascade" development. In this method, a developer material comprising relatively large carrier particles having finely-divided toner particles electrostatically clinging to the surface of the carrier particles is conveyed to and rolled or cascaded across the electrostatic latent image-bearing surface. The composition of the toner particles is so chosen as to have a triboelectric polarity opposite that of carrier particles. As the mixture cascades or rolls across the image-bearing surface, the toner particles are electrostatically deposited and secured to the charged portion of the latent image and are not deposited on the uncharged or background portions of the image. Most of the toner particles accidentally deposited in the background are removed by the rolling carrier, due apparently, to the greater electrostatic attraction between the toner and the carrier than between the toner and the discharged background. The carrier particles and unused toner particles are then recycled. This technique is extremely good for the development of line copy images. The cascade development process is the most widely used commercial electrostatographic development technique. A general purpose office copying machine incorporating this technique is described in U.S. Pat. No. 3,099,943.
Another technique for developing electrostatic latent images is the "magnetic brush" process as disclosed, for example, in U.S. Pat. No. 2,874,063. In this method, a developer material containing toner and magnetic carrier particles is carried by a magnet. The magnetic field of the magnet causes alignment of the magnetic carriers in a brush-like configuration. This "magnetic brush" is engaged with an electrostatic-image bearing surface and the toner particles are drawn from the brush to the electrostatic image by electrostatic attraction.
While ordinarily capable of producing good quality images, conventional developing materials suffer serious deficiencies in certain areas. The developing materials must flow freely to facilitate accurate metering and even distribution during the development and developer recycling phases of the electrostatographic process. Some developer materials, though processing desirable properties such as proper triboelectric characteristics, are unsuitable because they tend to cake, bridge and agglomerate during handling and storage. Adherence of carrier particles to reusable electrostatographic imaging surfaces causes the formation of undesirable scratches on the surfaces during image transfer and surface cleaning steps. The tendency of carrier particles to adhere to imaging surfaces is aggravated when the carrier surfaces are rough and irregular. The coatings of some carrier particles deteriorate rapidly when employed in continuous processes which require the recycling of carrier particles by bucket conveyors partially submerged in the developer supply such as disclosed in U.S. Pat. No. 3,099,943. Deterioration occurs when portions of or the entire coating separates from the carrier core. The separation may be in the form of chips, flakes or entire layers and is primarily caused by fragile, poorly adhering coating materials which fail upon impact and abrasive contact with machine parts and other carrier particles. Carriers having coatings which tend to chip and otherwise separate from the carrier core must be frequently replaced thereby increasing expense and consuming time. Print deletion and poor print quality occur when carrier having damaged coatings are not replaced. Fines and grit formed from carrier disintegration tend to drift and form unwanted deposits on critical machine parts. Many carrier coatings having high compressive and tensile strength either do not adhere well to the carrier core or do not possess the desired triboelectric characteristics. The triboelectric and flow characteristics of many carriers are adversely affected when relative humidity is high. For example, the triboelectric values of some carrier coatings fluctuate with changes in relative humidity and are not desirable for employment in electrostatographic systems, particularly in automatic machines which require carriers having stable and predictable triboelectric values. Another factor affecting the stability of carrier triboelectric properties is the susceptibility of carrier coatings to "toner impaction". When carrier particles are employed in automatic machines and recycled through many cycles, the many collisions which occur between the carrier particles and other surfaces in the machine cause the toner particles carried on the surface of the carrier particles to be welded or otherwise forced into the carrier coatings. The gradual accumulation of permanently attached toner material on the surface of the carrier particles causes a change in the triboelectric value of the carrier particles and directly contributes to the degradation of copy quality by eventual destruction of the toner carrying capacity of the carrier.
Heretofore, electrostatographic coated carrier particles have generally been prepared by solution, immersion, spray drying, and fluidized-bed coating methods. More particularly, by conventional methods electrostatographic carrier particles are coated by preparing a solution of the coating material in a solvent and contacting the carrier cores with the coating material by dipping the carrier cores in the coating solution; by spraying the carrier cores with a coating solution; and by creating a fluidized bed of carrier cores while contacting the carrier cores with a solution or dispersion of coating material. However, these known methods all suffer from various disadvantages. That is, in these techniques, it is usually very difficult to control the amount of coating material deposited on the carrier cores. Where a particular coating material would be desirable, its use may be precluded due to solubility considerations in preparing a coating solution. These processes necessarily require the use of a solvent which must be removed from the coated carrier surface thereby leading to contamination of the atmosphere by the vapors or requiring the use of expensive and elaborate equipment for its capture. Further, carrier beads having a solution of coating material on their surfaces have a tendency to agglomerate into large masses during the drying step. In addition, selection of a suitable solvent is difficult due to safety considerations; the incompatibility of the solvent with the carrier core surface may lead to poor adhesion of the coating material to the carrier surface and subsequent loss of the carrier coating resulting in poor performance of the developer mixture. Further, the use of solvents may dissolve carrier core surfaces making uniform surface coatings unattainable. Thus, there is a continuing need for a better method of preparing electrostatographic coated carrier materials.
It is, therefore, an object of this invention to provide a method for preparing electrostatographic coated carrier materials which overcomes the above noted deficiencies.
It is another object of this invention to provide a method of preparing electrostatographic coated carrier materials which avoids the need for the use of coating solutions.
It is a further object of this invention to provide a method of preparing electrostatographic coated carrier materials without the use of solvents.
It is still a further object of this invention to provide a method of coating electrostatographic carrier materials which permits the use of substantially any coating material.
It is yet another object of this invention to provide a method of coating electrostatographic carrier materials which avoids the need for removing solvents from coating solutions and the use of drying equipment.
It is yet another object of this invention to provide coated electrostatographic carrier materials having improved coatings.
It is another object of this invention to provide developer materials having physical and chemical properties superior to those of known developer materials.
The above objects and others are accomplished, generally speaking, by electrostatically attracting at least one coating material to a carrier core material and then heating the coated core material causing the coating material to fuse and adhere to the carrier core.
In one embodiment of this invention, various coating materials may be applied to carrier core materials by a continuous process wherein electrostatically charged carrier core particles are rolled down an inclined plane. As the charged particles move down the inclined plane, a spray of oppositely charged coating material in particle form is directed at the core particles. The coated core particles are then heated causing the coating material to fuse and adhere to the carrier core. After cooling, the coated carrier particles are ready for use and may be mixed with finely-divided toner particles to form developer mixtures.
In another embodiment of this invention, electrostatically charged carrier core particles are dropped into a "cloud chamber" containing coating material particles having an electrostatic charge opposite to that of the charged carrier core particles. The "cloud chamber" may generally be a chamber wherein charged coating material particles are suspended in air or a gas stream. As the charged carrier core particles pass through the cloud of charged coating material particles, the charged coating material particles are electrostatically attracted and electrostatically adhered to the charged carrier core particles. The thus coated carrier core particles are then heated whereby the coating material particles fuse and adhere to the carrier core forming coated carrier particles.
In yet another embodiment of the process of this invention, a coating material may be applied to carrier core particles by mixing or blending particles of a coating material with carrier core particles until the carrier core particles are uniformly coated with the coating material through electrostatic attraction. The coated carrier core particles are then heated and the coating material is fused to the carrier core particles.
Thus, in accordance with the process of this invention, various coating materials and mixtures of coating materials may be electrostatically attracted to and adhered to carrier core particles followed by heating whereby the coating materials fuse into a continuous coating over the carrier core particles to form coated electrostatographic carrier beads. The electrostatographic carrier beads formed by the process of this invention have good mechanical, thermal, and electrical properties and provide excellent results when employed in electrostatographic copying and duplicating devices.